In recent years, a weight reduction of a tire has been attempted in accordance with strong social requests with respect to a reduction of fuel consumption for a vehicle. Among tire members, a weight reduction has been attempted also for an inner liner which is arranged on an inner side of a tire in a radial direction and serves to improve an air permeation resistance by reducing an amount of leakage of air from inside to outside of a pneumatic tire (air permeation amount).
Currently, as a rubber composition for an inner liner, butyl-based rubber containing isobutylene-isoprene rubber by 70 to 100 mass % and natural rubber by 30 to 0 mass % is used to improve the air permeation resistance of a tire. Moreover, butyl-based rubber contains isoprene by about 1 mass % in addition to butylene, and it enables crosslinking with adjacent rubber cojointly with sulfur, vulcanization accelerator, and zinc white. The butyl-based rubber described above requires in a normal blending a thickness of about 0.6 to 1.0 mm for a tire of a passenger car, and a thickness of about 1.0 to 2.0 mm for a tire of a bus and a truck.
Therefore, it has been proposed to use thermoplastic elastomer, which is superior in an air permeation resistance than butyl-based rubber and is capable of reducing a thickness of an inner liner layer, for an inner liner to reduce a weight of a tire. However, the thermoplastic elastomer, which has a smaller thickness than butyl-based rubber and exhibits a high air permeation resistance, has less vulcanization adherence with insulation rubber or carcass rubber adjacent to the inner liner as compared to butyl-based rubber. When an inner liner has a low vulcanization adherence, an air-in phenomenon occurs in which air is added between an inner liner and an insulation or a carcass to cause a large numbers of small bubbles to appear. Since this phenomenon involves small dot patterns inside of a tire, it disadvantageously gives an impression to a user that an appearance is not good. Further, since an insulation or a carcass is peeled from an inner liner due to air as a starting point during traveling, a crack may occur in the inner liner to lower an internal pressure of the tire. Then, the tire may burst in the worst case.
In PTD 1 (Japanese Patent Laying-Open No. 9-165469), a pneumatic tire is proposed which can improve an adhesiveness between an inner liner and a rubber composition forming an inner portion of a tire or a carcass layer by forming the inner liner layer with use of nylon having a low air permeability. However, in this technique, it would be necessary to apply an RFL treatment to a nylon film and thereafter attach a rubber cement composed of a rubber composition to form a nylon film layer. Thus, steps are complicated disadvantageously.
Moreover, an inner liner portion of a pneumatic tire is likely to generate heat during traveling of a vehicle. Under a high-temperature condition, a stiffness is likely to be lowered, and an operation stability may be deteriorated. Thus, in view of the safety, an inner liner capable of improving the operation stability is also requested.
In PTD 2 (Japanese Patent Laying-Open No. 2011-057940), a tire for competition is proposed which can improve an operation stability by using a rubber composition including carbon black by 30 to 110 mass parts with respect to a rubber component by 100 mass parts. However, this technique should be further reviewed as to its air permeation resistance during continuous use for a long period of time.